Since nitride semiconductors have characteristics such as exhibiting high saturation electron velocity and a wide band gap, application of nitride semiconductors to semiconductor devices having high breakdown voltage and high power has been studied on the basis of utilization of such characteristics. For example, the band gap of GaN that is a nitride semiconductor is 3.4 eV and larger than the band gap of Si (1.1 eV) and the band gap of GaAs (1.4 eV); thus, GaN exhibits high breakdown field strength. GaN is therefore a highly practical material used for power semiconductor devices which operate at high voltage and which output high power.
Semiconductor devices utilizing nitride semiconductors, such as field effect transistors, have been reported, in particular, high electron mobility transistors (HEMTs). Among HEMTs utilizing GaN (GaN-HEMTs), for instance, an AlGaN/GaN-HEMT in which GaN is used for an electron transit layer and in which AlGaN is used for an electron supply layer is attracting attention. In the AlGaN/GaN-HEMT, strain is generated in AlGaN owing to difference in the lattice constant between GaN and AlGaN. The strain causes piezoelectric polarization, and the piezoelectric polarization and the spontaneous polarization difference between AlGaN and GaN cause high-concentration two-dimensional electron gas (2DEG). The AlGaN/GaN-HEMT is therefore expected to be applied to highly efficient switching devices and to power devices having high breakdown voltage and used in, for example, electric vehicles.
Related art is disclosed in Japanese Laid-open Patent Publication No. 2010-263011; O. Ambacher, J. Smart, J. R. Shealy, N. G. Weimann, K. Chu, M. Murphy, W. J. Schaff, L. F. Eastman, R. Dimitrov, L. Wittmer, M. Stutzmann, W. Rieger, and J. Hilsenbeck, Journal of Applied Physics, 85, 3222, 1999; and K. Matocha, T. P. Chow, and R. J. Gutmann, IEEE ELECTRON DEVICE LETTERS, VOL. 23, p79, 2002.
In an AlGaN/GaN-HEMT, a number of electrons are present in a channel, and electric current therefore flows in the channel even in a state in which a gate voltage is not applied; in other words, this phenomenon is operation in a normally-on mode. In order to interrupt this flow of electric current, a negative voltage is applied to a gate electrode.
In order to use GaN-HEMTs as power devices having high breakdown voltage, in terms of fail safe, GaN-HEMTs desirably operate in a normally-off mode in which electric current does not flow in channels in a state in which a gate voltage is not applied.
A metal oxide semiconductor (MOS) GaN-HEMT has been developed as a GaN-HEMT which may operate in a normally-off mode; in particular, GaN is used for an electron transit layer, and a gate electrode is formed above the electron transit layer with a gate insulating film interposed there between them. In GaN-HEMTs, GaN is generally used to form an electron transit layer of which a surface (upper surface) is the Ga-face that is the c-plane (0001). In this case, negative spontaneous polarization charges are generated in the vicinity of the interface of the electron transit layer to the gate insulating film.
Such spontaneous polarization charges significantly vary with an increase in temperature. Existing GaN-HEMTs have a problem of high temperature dependence of a threshold voltage.
FIG. 1A illustrates the C-V characteristics of an example of existing GaN-MOS diodes, and FIG. 1B illustrates the relationship (Measured Vfb) between a flat band voltage and temperature and a theoretical curve (Ideal Vfb) in the example of existing GaN-MOS diodes. As is clear from FIG. 1A, C-V curves shift to the right side with an increase in temperature. This indicates that negative spontaneous polarization charges increase with an increase in temperature. As illustrated in FIG. 1B, although temperature dependence of flat band voltage is theoretically very weak, temperature dependence of negative spontaneous polarization charges significantly enhances temperature dependence of a flat band voltage, which is problematic.